1. Field of the Invention
The present invention relates to an auto-focus apparatus, image capture apparatus, and auto-focus method for automatically adjusting focus on a subject using evaluation values generated by image data processing, so that the subject image is in focal status (hereinafter called “in-focus”).
2. Description of the Related Art
Typical image capture apparatus, such as video cameras or digital still cameras, includes an auto-focus or AF (automatically adjusting focus) unit for automatically adjusting focus to a subject. Japanese Unexamined Patent Application Publication No. 10-213736, for example, discloses examples of various technologies to improve an accuracy of auto-focus mechanism.
An outline of the typical focus control apparatus for video cameras according to the related art will be described below. FIG. 1 illustrates a configuration of a typical video camera according to the related art. The video camera shown in FIG. 1 performs auto-focus operation using evaluation values generated by image processing. A lens block of the video camera includes a lens group having an image capture lens 101c and a focus lens 101; a position detector 101a; a lens mechanism 10b; and a lens driver 102. The camera block further includes an image-capture device 103, an image-capture device driver 104, an image signal generator 105, an image signal processor 106, an evaluation value calculator 107, a control unit 109, a memory 110, and a switch 113.
A subject image to which focus has been adjusted by moving the focus lens 101 is formed on the image-capture device 103 such as CCD (Charge Coupled Devices) in this video camera. The subject image is then photo-electrically converted into electric signals by the image-capture device 103, and output the signals to the image signal generator 105. The focus lens 101 receives instructions from the lens driver 102 and is moved by the lens drive mechanism 101b. The lens driver 102 includes a lens CPU and a lens drive circuit and outputs instructions for moving the focus lens 101 and adjusting focus (focal point) based on the instructions from the control unit 109. Positions of the focus lens 101a or focus positions are detected by the position detector 101a. 
The image-capture device driver 104 drives the image-capture device 103 to photo-electrically convert the subject image formed on the image-capture device 103 and generate photo-electrically converted electric signals. The image signal generator 105 performs appropriate signal processing on electric signals output from the image-capture device 103 and generates image signals complied with a prescribed standard. The image signals are transmitted to a circuit group (image signal processor 106) while input to the evaluation value calculator 107. The evaluation value calculator 107 filters high frequency components of the image signals in a specific region defined within an imaging frame to calculate evaluation values relative to image contrast. When capturing an image of a typical subject, an evaluation value increases as the image approximates in-focus state, and the evaluation value is the relative maximum when the image is in-focus. The aforementioned evaluation value is updated once for one field of image signals.
The control unit 109 includes a CPU (Central Processing Unit), and the like, receives evaluation values calculated by the evaluation value calculator 107 once for one field and operates to search the peak of evaluation values.
A memory 110 includes a semiconductor memory, such as a RAM, and stores focus positions of the lens 101 and information such as evaluation values.
The switch 113 indicates one-shot switch to direct activation of auto-focus operation.
In the configuration of the aforementioned video camera, the control unit 109 moves the focus lens using the evaluation values obtained by image processing, and controls the evaluation values to reach the relative maximum (in-focus state); that is, the control unit 109 operates to search the peak of the evaluation values in order to acquire the relative maximum of the evaluation values. Accordingly, the peak of the evaluation values may be detected regardless of a high-contrast or low contrast subject image.